Portable Distributed Objects
Encyclopedia
Portable Distributed Objects, or PDO, is a programming API
for creating object-oriented code that can be executed remotely on a network
of computers. It was created by NeXT Computer, Inc. using their OpenStep
system, whose use of Objective-C
made the package very easy to write. It was characterized by its very light weight and high speed in comparison to similar systems such as CORBA
.
Versions of PDO were available for Solaris, HP-UX
and all versions of the OPENSTEP system. A version that worked with Microsoft
OLE
was also available called D'OLE, allowing distributed code written using PDO on any platform to be presented on Microsoft systems as if they were local OLE objects.
PDO was one of a number of distributed object
systems created in the early 1990s, a design model where "front end" applications on GUI
-based microcomputers would call code running on mainframe
and minicomputer
s for their processing and data storage. Microsoft was evolving OLE into the Component Object Model
(COM) and a similar distributed version called DCOM, IBM
had their System Object Model
(SOM/DSOM), Sun Microsystems
was promoting their Distributed Objects Everywhere
, and there were a host of smaller players as well. With the exception of the limited functionality in COM, most of these systems were extremely heavyweight, tended to be very large and slow, and often were very difficult to use.
PDO, on the other hand, relied on a small number of features in the Objective C runtime to handle both portability as well as distribution. The key feature was the language's support for a "second chance" method in all classes; if a method call on an object failed because the object didn't support it (normally not allowed in most languages due to strong typing), the runtime would then bundle the message into a compact format and pass it back into the object's
The normal behavior for
In comparison with CORBA
, PDO programs were typically 1/10 or less in size; it was common for NeXT staffers to write into magazines showing how to re-implement a multi-page CORBA article in perhaps 15 lines of code. From a programming standpoint, there was nearly nothing as easy to use as PDO.
However, PDO was also reliant entirely on Objective C to function. This was a price most were unwilling to pay, as at the time C++
was more widely used and the effort to shift codebases to an entirely new language and paradigm was considered too onerous. PDO never saw much use, and NeXT's emphasis shifted to its new WebObjects
framework in 1995.
PDO continues to be used by Mac OS X
programmers as a method for interprocess and interapplication communication, and for communication between networked applications that only need compatibility with other Mac OS X applications.
In addition to the OS X platform, there is GNUstep
, which has its own implementation of Distributed Objects.
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
for creating object-oriented code that can be executed remotely on a network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
of computers. It was created by NeXT Computer, Inc. using their OpenStep
OpenStep
OpenStep was an object-oriented application programming interface specification for an object-oriented operating system that used a non-NeXTSTEP operating system as its core, principally developed by NeXT with Sun Microsystems. OPENSTEP was a specific implementation of the OpenStep API developed...
system, whose use of Objective-C
Objective-C
Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
made the package very easy to write. It was characterized by its very light weight and high speed in comparison to similar systems such as CORBA
Çorba
Chorba , ciorbă , shurpa , shorpo , or sorpa is one of various kinds of soup or stew found in national cuisines across Middle East...
.
Versions of PDO were available for Solaris, HP-UX
HP-UX
HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...
and all versions of the OPENSTEP system. A version that worked with Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
OLE
Object Linking and Embedding
Object Linking and Embedding is a technology developed by Microsoft that allows embedding and linking to documents and other objects. For developers, it brought OLE Control eXtension , a way to develop and use custom user interface elements...
was also available called D'OLE, allowing distributed code written using PDO on any platform to be presented on Microsoft systems as if they were local OLE objects.
PDO was one of a number of distributed object
Distributed object
The term distributed objects usually refers to software modules that are designed to work together, but reside either in multiple computers connected via a network or in different processes inside the same computer. One object sends a message to another object in a remote machine or process to...
systems created in the early 1990s, a design model where "front end" applications on GUI
Gui
Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...
-based microcomputers would call code running on mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
and minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
s for their processing and data storage. Microsoft was evolving OLE into the Component Object Model
Component Object Model
Component Object Model is a binary-interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages...
(COM) and a similar distributed version called DCOM, IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
had their System Object Model
System Object Model
In computing, the System Object Model is an object-oriented shared library system developed by IBM. DSOM, a distributed version based on CORBA, allowed objects on different computers to communicate.-Applications:...
(SOM/DSOM), Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
was promoting their Distributed Objects Everywhere
Distributed Objects Everywhere
Distributed Objects Everywhere was a long-running Sun Microsystems project to build a distributed computing environment based on the CORBA system in the 'back end' and OpenStep as the user interface. First started in 1990 and announced soon thereafter, it remained vaporware for many years before...
, and there were a host of smaller players as well. With the exception of the limited functionality in COM, most of these systems were extremely heavyweight, tended to be very large and slow, and often were very difficult to use.
PDO, on the other hand, relied on a small number of features in the Objective C runtime to handle both portability as well as distribution. The key feature was the language's support for a "second chance" method in all classes; if a method call on an object failed because the object didn't support it (normally not allowed in most languages due to strong typing), the runtime would then bundle the message into a compact format and pass it back into the object's
forwardInvocation method.The normal behavior for
forwardInvocation was to return an error, including details taken from the message (the "invocation"). PDO instead supplied a number of new objects with forwardInvocation methods that passed the invocation object to another machine on the network, with various versions to support different networks and platforms. Calling methods on remote objects was almost invisible; after some network setup (a few lines typically) PDO objects were instantiated locally and called the same way as any other object on the system. The PDO object then forwarded the invocation to the remote computer for processing and unbundled the results when they were returned.In comparison with CORBA
Çorba
Chorba , ciorbă , shurpa , shorpo , or sorpa is one of various kinds of soup or stew found in national cuisines across Middle East...
, PDO programs were typically 1/10 or less in size; it was common for NeXT staffers to write into magazines showing how to re-implement a multi-page CORBA article in perhaps 15 lines of code. From a programming standpoint, there was nearly nothing as easy to use as PDO.
However, PDO was also reliant entirely on Objective C to function. This was a price most were unwilling to pay, as at the time C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
was more widely used and the effort to shift codebases to an entirely new language and paradigm was considered too onerous. PDO never saw much use, and NeXT's emphasis shifted to its new WebObjects
WebObjects
WebObjects was a Java web application server from Apple Inc., and a web application framework that ran on the server. It was available at no additional cost. Its hallmark features were its object-orientation, database connectivity, and prototyping tools...
framework in 1995.
PDO continues to be used by Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
programmers as a method for interprocess and interapplication communication, and for communication between networked applications that only need compatibility with other Mac OS X applications.
In addition to the OS X platform, there is GNUstep
GNUstep
GNUstep is a free software implementation of Cocoa Objective-C libraries , widget toolkit, and application development tools not only for Unix-like operating systems, but also for Microsoft Windows. It is part of the GNU Project.GNUstep features a cross-platform, object-oriented development...
, which has its own implementation of Distributed Objects.
External links
- PDO History
- Distributed Objects — a guide by AppleApple ComputerApple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...